Fast-neutron nuclear reactors cooled by a liquid metal, such as sodium, comprise a core consisting of prism-shaped, elongate assemblies submerged in liquid sodium contained in the reactor vessel.
It may be necessary to extract fuel assemblies from the core of the reactor within the vessel, for example in order to replace spent or defective assemblies by new assemblies. The assemblies which are removed from the reactor vessel are in an irradiated state and generate radioactive radiation. Furthermore, these assemblies have residual activity and give rise to thermal emission caused by nuclear reactions in the irradiated fuel. Finally, these assemblies, at the moment when they are removed from the vessel, retain some liquid sodium.
The fuel assemblies removed from the vessel of the nuclear reactor therefore have to be arranged in containers ensuring the biological protection of the environment in which the assemblies are moved or temporarily stored. It is also necessary to cool the assembly during its transfer.
To carry out the transfer and temporary storage of the fuel assemblies of fast-neutron nuclear reactors, it is customary to use cupboards comprising a solid body made of a material absorbing nuclear radiation, in which is provided a receptacle for a fuel assembly exposed or immersed in a casing filled with sodium to ensure its cooling. Fuel assemblies for fast-neutron reactors are prism-shaped and are of great length in relation to their transverse dimensions. The body of the cupboard therefore has an elongate shape and the receptacle of the fuel assembly is arranged in the solid body, in a central position and in a substantially axial direction.
The receptacle opens out at one of the ends of the cupboard body via a sealingly closable orifice, so as to make it possible to introduce the assembly into the cupboard and extract it. These operations are conducted by means of a lifting assembly comprising winches, associated with the cupboard: in general, the device for closing the cupboard is connected to a corresponding device associated, for example, with a passage well for the assemblies.
To ensure that the assembly is cooled during its transfer, an assembly of circuits and of appliances making it possible to circulate a neutral cooling gas in contact with the fuel assembly arranged in the cupboard receptacle and to cool and purify this circulating gas is used.
The assembly of circuits and of appliances necessary for these operations, comprising particularly blowers, heat exchangers and gas purifiers, for example cyclone separators, is installed outside the cupboard, with the result that the total volume of the installation is large and that this installation does not afford all the necessary safety guarantees; for example, atmospheric oxygen is liable to mix with the cooling gas if one of the circuits or appliances outside the cupboard has a leak. Moreover, radioactive fluids are made to circulate on the outside of the body of the cupboard ensuring biological protection.